A device of this sort is used especially as a television back projector; the matrices of electrically driveable reflecting elements may, for example, be produced from:                electrooptic modulators operating in reflection, based on liquid crystals (LC), especially liquid crystals applied on a silicon substrate (LCOS or “Liquid Crystal On Silicon”);        electrooptic modulators based on matrices of micromirrors, called “DMD” or “Digital Mirror Device”.        
In general, the matrices MB, MG and MR are arranged so that the planes of their reflecting surface intersect along parallel straight lines; moreover, these reflecting surfaces are generally vertical and mutually orthogonal.
Conventionally, as illustrated in FIG. 1, the means 2 for deconstructing the beam BS of polychromatic light and/or the means 3 for reconstructing the reflected complementary beams B′B, B′G and B′R each comprise two dichroic filters 21, 22; 21′, 22′ arranged at a predetermined mean angle of incidence β with respect to the optical axis of the incident beams to be deconstructed and/or reconstructed, each dichroic filter 21, 22; 21′, 22′ having, for this angle of incidence β, its cutoff wavelength matched, in a manner known per se, to deconstruct or reconstruct this or these incident beams; each filter generally being rectangular, the envelope of these two dichroic filters then forms a parallelepiped; the predetermined angles of incidence of these filters are generally about β=45° or 135°, such that the two filters 21, 22; 21′, 22′ are in general arranged orthogonally with respect to each other, as shown in FIG. 1.
On the side of the complementary beams BB, BG and BR and/or B′B, B′G and B′R, it is possible to place filters, called confirmation filters, FB, FG and FR on the one hand, F′B (not shown), F′G and F′R on the other hand.
As shown in FIG. 1, the dichroic filters 21, 22; 21′, 22′ are placed on the vertical diagonals of the parallelepipeds and the confirmation filters FB, FG, FR; F′B, F′G, F′R are placed on the vertical walls of these parallelepipeds; FIG. 4, which shows a partial bottom view of the display device where only the filter 22 of the deconstruction means 2 has been represented, clearly shows that this filter is placed along the diagonal of the parallelepiped; in this case, since the horizontal cross section of this parallelepiped is square, the angle of incidence β, formed at the centre O of the filter 22 by the optical axis of the polychromatic beam BS and the surface of this filter 22 is in this case 45°.
The longest dimension of the filters 21, 22; 21′, 22′ (the longest side of the rectangle) corresponds to the longest dimension of the matrices MB, MG and MR of reflecting elements and the longest dimension of the images to be displayed; if the optical axis of each incident beam strikes the dichroic filter at a midpoint of incidence O and forms, at this point, an angle β=45° with the plane of this filter, the rays of this beam which strike the filter at points other than this midpoint of incidence O have angles of incidence which vary around this mean value of 45° (or of 135°); the variation of the angles of incidence is obviously greatest along the longest dimension of the filter.
Since the cutoff wavelength of a dichroic filter depends on the angle of incidence, many defects in beam deconstruction and/or reconstruction and chromatic defects would be obtained with a conventional dichroic filter.
To prevent these defects, it is known to use dichroic filters with a gradient, which have a constant cutoff wavelength along a direction parallel to their longest dimension located in a plane orthogonal to the reflecting surface of the matrices MB, MG and MR; this arrangement of the filters and this orientation of the gradient is perfectly matched to obtain a constant cutoff wavelength for all the rays of the beam located in this orthogonal plane; the direction of the index gradient of the layers of these filters is thus parallel to the longest dimension of these filters and included in this orthogonal plane.
As illustrated in FIG. 3, which shows a partial schematic side view of the display device, since the matrices MB, MG (shown alone) and MR for modulating the complementary beams operate by reflection, the angle of incidence α of the optical axis of each incident beam BB, BG and BR respectively on each matrix MB, MG and MR is different from the normal to these matrices, so that the incident beams BB, BG and BR coming from the source 1 from the reflected beams B′B, B′G and B′R directed towards the projection objective 5 can be properly separated; because the angle of incidence on each matrix MB, MG and MR is different from the normal to these matrices, and because the deconstruction means 2 and the reconstruction means 3 are superimposed, the optical axis common to the beams BS and BG makes an angle of 2×α with the optical axis common to the beams B′G and B′P reflected on the matrix MG; the value of the angle α depends on the dimensions and on the arrangement of the optical components of the display device; this angle α is generally between 5° and 20°; by way of example, in this case, this angle is 12°5.
FIG. 5 shows a perspective view of the dichroic filter 22 (the shaded part in the figure) and of the optical axis of the polychromatic beam BS coming from the source S and passing through this filter at O; the projection of the central point S of the source onto a plane normal to the filter 22, intersecting it along a secant DOE passing through O, is called T; the projection of this same point S onto the plane of the filter 22 is called Q; also, the common projection of the point Q and of the point T onto the secant DOE is called R; it will be immediately deduced that, in the horizontal plane, the angle ROT=γ=90°−ββ45° and that, in the vertical plane, the angle TOS=α.
FIG. 6 shows, in a manner comparable to that of FIG. 5, the same rectangular dichroic filter 22; in this figure, the rays SAM, SOP and SCN are defined as forming a horizontal median line on the matrix MG; it will be seen that, as indicated above, SA, SO and SC of the same beam BS which strike the filter at different points A, O and C, have angles of incidence which vary around a mean value; in this case, 35.55°, 46.35°, 56.55° respectively for a distance OA=OC=20 mm.
Now, at the midpoint of incidence O of the filter, the cutoff lengths are set for a predetermined angle of incidence of 45°; because of the non-zero angle of incidence α=12°5 on the matrices MB, MG and MR, the difference in the angle of incidence (46.35° compared with β=45°) observed at the midpoint of incidence O of the filter compared to the predetermined angle of incidence β=45° leads to a detrimental shift in the cutoff wavelengths of the filter.
For the other points of incidence away from the midpoint of incidence O of the filter, especially the points of incidence such as A and C of the rays included in the longest dimension of the intersection of the incident beam BS with the filter 22, the direction of the filter gradient is not properly matched; this is because, since the filter gradient in this case extends in a conventional manner along a direction DOE parallel to the longest dimension of this filter DOE which does not correspond to that of the longest dimension AOC of the intersection of the incident beam BS with the filter 22 since the angle α is not zero, the gradient no longer corresponds to the distribution of angles of incidence for which the cutoff wavelengths remain constant; in other words, the filter gradient, which is matched to obtain constant cutoff wavelengths along the straight midline DOE is not matched in order to obtain constant cutoff wavelengths along the straight line AOC.
Thus, not only at the midpoint of incidence O of the filter, but along the entire longest dimension of the intersection of the incident beam BS with the filter, in this case the straight line AOC, the fact that the angle of incidence α on the matrices MB, MG and MR is not zero leads, along this entire straight line AOC, to a difference between the actual angles of incidence and the ideal angles of incidence for which, by constructing the dichroic filter with a gradient, the cutoff wavelengths are constant; in spite of using a filter with a gradient, the fact that the angle α is not zero therefore leads to a detrimental shift in the cutoff wavelengths of the dichroic filters or of the deconstruction means 2, or of the reconstruction means 3, or even of both; this shift is detrimental since it leads to chromatic defects on the displayed image.
The aim of the invention is to prevent, or at least, to limit this drawback.